The present invention relates to sensors for use in sensing properties of industrial fluids for process control. In particular, the present invention relates to sensors that include isolation between the industrial fluids and the sensor""s leads.
Industrial transmitters are used to sense fluid parameters such as pressure, temperature, flow, and pH, and transmit the value of the sensed fluid parameter to a remote location such as a control system. These transmitters include sensors that are mounted in or near the transmitter. The sensor has a sensing surface that contacts an industrial fluid, and also electrical sensor leads. Various types of isolation are used to ensure accurate, reliable operation of each sensor. The electrical sensor leads are fluid-isolated from contact with the industrial fluid to avoid corroding the leads. The sensing surface is mechanically isolated from mounting stress to reduce errors in the sensor""s electrical output due to mechanical stress, particularly at temperature extremes. The sensor""s electrical circuitry is galvanically isolated from the industrial fluid to avoid errors from stray ground current. These three types of isolation need to be robust and effective over a wide operating temperature range for use in an industrial transmitter.
It is difficult to mount a sensor in a transmitter in a way that provides corrosion resistance for the sensor leads, adequate mechanical stress isolation, and also galvanic isolation. The problem is aggravated by wide operating temperature ranges for industrial transmitters. As temperature increases, expensive glass-to-metal seals tend to leak, mounting components expand at different rates leading to mechanical stress, and galvanic leakage increases with temperature.
In particular, when a sensor is extremely miniaturized and has rectangular cross sectional dimensions on the order of 5 mm (0.2 inch), it is difficult to precisely machine a correspondingly rectangular opening in a sensor mounting block which fits to the miniature sensor cross section with a small gap that can be reliably sealed.
A sensor capsule is needed that provides stress isolation, fluid isolation and electrical isolation in a package that is sized for a miniature sensor and suitable for use in an industrial transmitter.
A sensor capsule suitable for use in an industrial process fluid transmitter is disclosed.
The sensor capsule comprises a block that has a sensor mounting hole with a sensor mounting hole length extending from a first outer block surface into a pressure chamber in the block. The block further includes a fluid inlet hole extending from a second outer block surface into the pressure chamber. The block comprises two half-blocks joined along joining or mating surfaces passing lengthwise through the sensor mounting hole.
The sensor capsule also includes a sensor that has a sensor neck passing through the sensor mounting hole. The sensor has a fluid sensing surface suspended in the pressure chamber and electrical sensor connections outside the pressure chamber. The sensor neck is separated from the sensor mounting hole by a gap that extends in a continuous path around the sensor neck. A sealing joint fills the gap to join the sensor neck to the sensor mounting hole.
These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings.